This invention relates generally to flowmeters of the vortex-shedding type which includes a deflectable section excited into vibration by fluidic oscillations, and more particularly to a vortex meter in which the vibrations of the deflectable section are picked up by a sensor which makes no physical contact therewith.
In many industrial processes, one must be able to measure the volumetric flow of fluids being treated or supplied in order to carry out various control functions. It is also necessary, in some instances, to determine the mass flow of the fluids. Existing types of vortex flowmeters are capable of effecting volumetric flow or mass flow measurement.
It is well known that under certain circumstances the presence of an obstacle in a flow conduit will give rise to periodic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed. These patterns are known as Karman vortex streets. The frequency at which vortices are shed in a Karman vortex street is a function of flow rate. In order to convert a volumetric reading to a reading of mass flow, one must multiply the volume measurement by the density of the fluid being measured.
An improved form of vortex-type flowmeter is disclosed in Burgess U.S. Pat. No. 3,589,185 wherein the signal derived from the fluid oscillation is relatively strong and stable to afford a favorable signal-to-noise ratio insuring accurate flow-rate information over a broad range. In this meter, the obstacle assembly mounted in the flow conduit is constituted by a block positioned across the conduit with its longitudinal axis at right angles to the direction of fluid flow, a strip being similarly mounted behind the block and being spaced therefrom to define a gap which serves to trap Karman vortices and to strengthen and stabilize the vortex street. This vortex street is sensed to produce a signal whose frequency is proportional to flow rate.
In a later Burgess U.S. Pat. No. 3,888,120, the disclosure of which is incorporated herein by reference, the obstacle assembly is formed by a front section mounted across the flow tube and a rear section cantilevered from the front section by means of a flexible beam to define a gap serving to trap the Karman vortices. Because the rear section is deflectable, it is excited into mechanical vibration by the vortices at a rate whose frequency is proportional to fluid flow. This vibratory motion is sensed by a strain-gauge sensor.
In the Vortex Flowmeter Model 10 LV 1000 manufactured by the Fischer & Porter Company of Warminster, Pa., the assignee of Burgess U.S. Pat. No. 3,888,120 as well as of the present application, a DSC-6 strain-gauge cartridge is used to sense the deflection of a T-shaped rear section in relation to the fixed front section of an obstacle assembly mounted in a flow tube. The DSC-6 strain-gauge sensor is constituted by a steel beam having a pair of high-impedance, semi-conductor strain gauges glass-bonded thereto. The characteristics of these gauges are such as to give rise to resistance changes of 0.66% for a 0.001 inch deflection at the tip of the cartridge.
While a strain-gauge sensor of the DSC-6 type is highly sensitive and operates effectively in vortex-shedding flowmeters, these sensors possess certain practical disadvantages.
In the manufacture of a strain-gauge sensor, it is important when bonding the gauges to the steel beam to avoid any physical deformation of the gauges, for such deformation acts to pre-stress the gauges and to impair their performance characteristics. It must be borne in mind that the deflectable section of the flowmeter, in the course of vibration, subjects the strain gauges to millions of vibratory cycles. Indeed, the Burgess U.S. Pat. No. 3,888,120 points out that in one example of a vortex flowmeter in continuous operation, the number of vibratory cycles per year is about 700 million.
In order, therefore, for the strain gauge to have an adequate fatigue life and to avoid overstressing effects that might result in the destruction of the strain gauge, great care must be exercised in bonding the gauge to the steel beam and in installing the strain gauge sensor in the meter to prevent gauge deformation. This adds substantially to manufacturing and installation costs.
Another drawback of strain gauge sensors is that in meters for measuring fluids such as liquid air or liquid nitrogen whose temperatures lie in the cryogenic range, such sensors are rendered inoperative by the extreme cold. Still another practical disadvantage is that semi-conductor strain gauge sensors have a relatively high impedance and the circuits associated therewith therefore tend to pick up extraneous noise voltages which degrade the performance of the measuring system.